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From "Hairong Kuang (JIRA)" <j...@apache.org>
Subject [jira] Updated: (HADOOP-5734) HDFS architecture documentation describes outdated placement policy
Date Mon, 27 Apr 2009 20:40:30 GMT

     [ https://issues.apache.org/jira/browse/HADOOP-5734?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
]

Hairong Kuang updated HADOOP-5734:
----------------------------------

    Fix Version/s: 0.21.0
     Hadoop Flags: [Reviewed]

+1. The change looks good to me.

> HDFS architecture documentation describes outdated placement policy
> -------------------------------------------------------------------
>
>                 Key: HADOOP-5734
>                 URL: https://issues.apache.org/jira/browse/HADOOP-5734
>             Project: Hadoop Core
>          Issue Type: Bug
>          Components: documentation
>    Affects Versions: 0.20.0
>            Reporter: Konstantin Boudnik
>            Priority: Minor
>             Fix For: 0.21.0
>
>         Attachments: HADOOP-5734.patch
>
>
> The "Replica Placement: The First Baby Steps" section of HDFS architecture document states:
> "...
> For the common case, when the replication factor is three, HDFS's placement policy is
to put one replica on one node in the local rack, another on a different node in the local
rack, and the last on a different node in a different rack. This policy cuts the inter-rack
write traffic which generally improves write performance.
> ..."
> However, according to the ReplicationTargetChooser.chooseTarger()'s code the actual logic
is to put the second replica on a different rack as well as the third replica. So you have
two replicas located on a different nodes of remote rack and one (initial replica) on the
local rack's node. Thus, the sentence should say something like this:
> "For the common case, when the replication factor is three, HDFS's placement policy is
to put one replica on one node in the local rack, another on a node in a different (remote)
rack, and the last on a different node in the same remote rack. This policy cuts the inter-rack
write traffic which generally improves write performance."

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